Problem: On the first 4 quizzes of his chemistry class, Brandon got an average score of 81. What does he need on the next quiz to have an overall average of 83?
Explanation: Let his score on the next quiz be $x$ The sum of all of his scores is then $4 \cdot 81 + x$ The same sum must also be equal to $5 \cdot 83$ Solve: $x = 5 \cdot 83 - 4 \cdot 81 = 91$.